Clamp-on ultrasonic flow measuring devices are often applied in process and automation technology. They permit volume and/or mass flow of a medium in a pipeline to be determined contactlessly, since they are placed externally on the pipeline. Clamp-on ultrasonic flow measuring devices are described, for example, in European Patent EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593. Usually, the ultrasonic sensors are secured individually with bands onto the pipeline, in which the flow is to be ascertained.
Ultrasonic flow measuring devices available from Endress+Hauser work according to the travel-time difference principle. In the travel-time difference principle, the different travel times of ultrasonic measurement signals in, and opposite to, the flow direction of the medium are evaluated. For this, the ultrasonic measuring signals of the ultrasonic sensors are alternately transmitted or received in the flow direction and counter to the flow direction of the medium. On the basis of the travel-time difference of the ultrasonic measuring signals, the flow velocity and therewith, in the case of known diameter of the tube, the volume flow, e.g. volume flow rate, or in the case of known density of the medium, the mass flow, can be determined.
In the case of clamp-on ultrasonic flow measuring devices, which work according to the travel-time difference principle, the ultrasonic measuring signals are radiated at a predetermined angle into the pipeline, in which the medium is located. In order that as large a part as possible of the energy radiated by an ultrasonic transducer into the containment be received by the other ultrasonic sensor, the two ultrasonic sensors must have a defined separation from one another. The particular positions of the ultrasonic sensors on the pipeline depend on the inner diameter of the pipeline and on the velocity of sound in the medium. As other application parameters, which occasionally lead to relatively large measurement errors, the wall thickness of the pipeline and the velocity of sound in the material of the pipeline can be named.
Added to this is the fact that known clamp-on ultrasonic flow measuring devices cannot be mounted on a pipeline without additional mounting aids, e.g. a screw driver. Furthermore, as already stated, the positioning and subsequent affixing of the ultrasonic sensors in the optimal in/out coupling regions is quite problematic.